For purposes of communicating well fluid to a surface of a well, the well may include a production tubing. More specifically, the production tubing typically extends downhole into a wellbore of the well for purposes of communicating well fluid from one or more subterranean formations through a central passageway of the production tubing to the well's surface. Due to its weight, the column of well fluid that is present in the production tubing may suppress the rate at which the well fluid is produced from the formation. More specifically, the column of well fluid inside the production tubing exerts a hydrostatic pressure that increases with well depth. Thus, near a particular producing formation, the hydrostatic pressure may be significant enough to substantially slow down the rate at which the well fluid is produced from the formation.
For purposes of reducing the hydrostatic pressure and thus enhancing the rate at which fluid is produced, an artificial lift technique may be employed. One such technique involves injecting gas into the production tubing to displace some of the well fluid in the tubing with lighter gas. The displacement of the well fluid with the lighter gas reduces the hydrostatic pressure inside the production tubing and allows reservoir fluids to enter the wellbore at a higher flow rate. The gas to be injected into the production tubing typically is conveyed downhole via the annulus (the annular space surrounding the production tubing) and enters the production tubing through one or more gas lift valves.
As an example, FIG. 1 depicts a prior art gas lift system 10 that includes a production tubing 14 that extends into a wellbore. For purposes of gas injection, the system includes a gas compressor 12 that is located at the surface of the well to pressurize gas that is communicated to an annulus 15 of the well. To control the communication of gas between the annulus 15 and a central passageway 17 of the production tubing 14, the system may include several side pocket gas lift mandrels 16 (gas lift mandrels 16a, 16b and 16c depicted as examples). Each of the gas lift mandrels 16 includes an associated gas lift valve 18 (gas lift valves 18a, 18b and 18c depicted as examples) for purposes of establishing one way fluid (gas) communication from the annulus 15 to the central passageway 17. As is well known, the gas lift valves 18a, 18b and 18c are commonly installed and retrieved from mandrel side pockets, such as by using a wireline and kickover tool inserted within the production tubing 14.
The gas lift valve 18 typically contains a check valve arrangement having a check valve element that opens to allow fluid flow from the annulus 15 into the production tubing 14 and closes when the fluid would otherwise flow in the opposite direction. Thus, when the pressure in the production tubing 14 exceeds the annulus pressure, the valve element is closed to ideally form a seal to prevent any reverse flow from the tubing 14 to the annulus 15. The prior art check valve arrangements are defined essentially by a single pair of sealing surfaces. One of the sealing surfaces belongs to a seat which is generally fixed in a housing or the like. The other sealing surface belongs to a valve element that is typically spring biased and moved back and forth in and out of engagement with the seat to close and open the check valve arrangement depending on a fluid pressure differential. The valve element could be a ball, a dart (or poppet), a flapper, a diaphragm, etc. In certain high temperature working conditions such as in an oil well environment, it is common to use dart-type check valve arrangements where substantially only metal-to-metal sealing elements are used. Metal-to-metal sealing is mainly dependent on conformity between sealing surfaces, surface finish, and contact stresses. Contact stresses are functions of applied pressure and contact area. The present inventors have found that a challenge can arise when a particular check valve arrangement is required to perform steadily at low back pressures and over a wide range of back pressures. If the contact area is too small once the valve is subject to high pressure, it is plastically or non-reversibly deformed. If the contact area is too large, the valve arrangement can experience low contact stresses at low pressure and thus will not seal.